Given a -current NetBSD box, with -current pkgsrc. USE_DESTDIR=yes is now
the default? So a
cd /usr/pkgsrc/meta-pkgs/gnome
make package
could fill up /usr/pkgsrc/packages with plenty of binary packages ready
for installation elsewhere, but not install gnome on the build box?
(Just using all the default variable settings.)

Yes. But asking to make a package is not what you want; you want to ask
that the package be made and installed:
make package-install

But that was right - I didn't want to install the package on the build
box. So here we go - cpu cycle burning time :-)

Each package's dependencies will be installed though (which in the case
of a meta package is ~the whole thing).